Characterized by a hardening of the cardiovascular arteries, arteriosclerosis is a medical condition that affects many patients. Fortunately, using medical procedures such as Percutaneous Transluminal Angioplasty (PTA), a sufficient flow of blood can be restored by implanting a tiny mesh tubular structure called a stent inside the affected lumen. In a typical PTA procedure, a stent is crimped about an inflatable balloon attached to the distal end of a catheter, and the catheter is introduced into a patient's vasculature. The catheter's distal end is maneuvered to a site of stenosis, where the balloon is inflated to expand the stent, compress the stenosis, and widen the lumen. The catheter is withdrawn after deflating the balloon.
Normally, the procedure restores a sufficient blood flow, but over time, the flow of blood may again be restricted by vascular problems, such as restenosis, which occur at or near the treatment site.
Restenosis is the renarrowing of a lumen following treatment. A common type of restenosis, known as the “candy wrapper effect”, takes its name from stenotic overgrowths appearing at the ends of implanted radioactive stents that remind viewers of a candy wrapper twist-tied about a piece of candy. Typically, the stenotic overgrowths occur within about 2.0 mm of the stent ends. It is believed that the candy wrapper effect is caused in part by balloon expansion, which may injure the lumen, and by a rapid decline in therapeutic radiation levels at the stent ends to a level that no longer prevents or inhibits restenosis. The minimal radiation level proven to prevent or inhibit restenosis is called the threshold level—a radiation dosage below this level being referred to as sub-threshold or sub-therapeutic. The threshold level is estimated to be in the range of approximately 0.5 microcuries and approximately 4.0 microcuries.
Stents may be of various types. Those that are crimped about a balloon and expanded by inflating the balloon are called balloon-expandable stents. Those that are crimped about a balloon and expanded by inflating the balloon with a warm or hot liquid are called thermal self-expanding stents. And, those that are compressed within a tubular sleeve and expanded by withdrawing the tubular sleeve are called self-expanding stents.
Regardless of how deployed, stents may be made of metals, metal alloys, polymers, biodegradable materials, and/or composites thereof. Manufacturing processes such as plasma ion deposition and plasma ion implantation may be used to make stents radioactive and/or radio-opaque. Additionally, stents may be made drug-eluting by forming pores in the material(s) comprising the stent and filling the pores with a drug that performs anti-proliferative, anti-platelet, or anti-clotting functions. For some applications, bioactive materials such as fibronectin, laminin, elastin, collagen, and integregrin may be incorporated into stents. In one process, applying radiation or drugs to stents involves preparing a solution containing the desired therapeutic substance and spraying the solution onto a horizontally positioned, rotating stent via an airbrush that moves laterally back and forth along the length of the rotating stent. In another process, radioactive or drug-eluting stents may be manufactured by affixing hollow or solid biodegradable fibers made of, filled with, or coated with therapeutic agents to the main body of the stent using heat pressing, extrusion molding, solvent heating, and similar attachment methods.
Two problems unsolved by prior approaches to stent design are a drastic tapering of radiation and drug dosage levels at stent ends and difficulty forming elastomeric bands or strips containing desired therapeutic agents in situ about the main body of a stent. Thus, a need exists for a composite stent providing a mechanism for increasing the dosages of drugs and radiation at the stent ends, and for a method providing a procedure for forming elastomeric bands or strips containing desired therapeutic agents in situ about the main body of a stent.